The availability of fresh water for drinking, irrigation and industrial applications is a critical issue facing the world given the impacts of climate change and the growing world population. The growth in world population coupled with greater environmental pollution has placed great strain on the availability of suitable amounts of fresh water and efforts have been made to improve the production of fresh water of this type.
As a result of these challenges significant research effort has been put into water purification technology such as desalination technology which can be applied to the purification of brackish ground water such as bore water and also to water obtained from oceans (salt water). One approach to the production of clean water involves using membrane technology which has been demonstrated as an economical and an energy efficient alternative to current technologies such as thermal distillation. The most commonly used membranes for desalination are thin film composite membranes (TFC) consisting of an active polyamide layer, which provides ion selectivity, deposited onto a porous support such as a polysulfone. The polyamide layer is fabricated through the rapid reaction between an aromatic di-functional amine and a tri-functional acid chloride, producing a negatively charged and heavily cross-linked film, which are the key properties leading to its high salt rejection performance. These membranes can be used in reverse osmosis, nanofiltration and forward osmosis applications.
The major drawback of the polyamide systems of this type is that the membranes are not tolerant to oxidizing agents such as chlorine. Water treatment systems are also susceptible to fouling by biological species and the current method of inhibiting such fouling involves the chlorination of the feed water supply. Unfortunately, the exposure of polyamide membranes to aqueous chlorine has been shown to cause degradation of the aromatic amide bonds, rapidly compromising membrane integrity. As a result most commercial membrane manufacturers recommend a near-zero exposure of the polyamide membranes to chlorinating agents to prolong membrane lifespan. Current solutions to this issue involve the chlorination of feed water to eliminate fouling species followed by de-chlorination upstream of the membrane system. Purified water is then re-chlorinated downstream for distribution to its point of use. The development of desalination membranes that are chlorine resistant would eliminate these additional processing steps, dramatically reducing operating costs.
One attractive approach to the formation of membranes of this type is the layer-by-layer (LbL) assembly of polyelectrolytes which allows for the fabrication of separation membranes whereby a wide range of polymers can be readily deposited in aqueous media with a high level of control of properties such as membrane porosity, layer thickness and composition. These polyelectrolyte multilayer (PEM) membranes have been studied extensively for nanofiltration, forward osmosis and ion selection. Unfortunately to date this methodology has not proven successful as the low rejection of monovalent ions has been a challenge for the use of PEM membranes in reverse osmosis (RO) and forward osmosis (FO) applications. Nevertheless this synthetic technique is attractive due to its relative simplicity which allows for the rapid and flexible synthesis of a wide variety of membrane materials.
Accordingly it would be desirable provide improved membrane materials that could be fabricated using LbL assembly techniques that could be used for applications of this type as it would be expected that they would find widespread application in water purification.
The discussion of documents, acts, materials, devices, articles and the like is included in this specification solely for the purpose of providing a context for the present invention. It is not suggested or represented that any or all of these matters formed part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed in Australia or elsewhere before the priority date of each claim of this application.